MEMS Microphone Cantilever Segmentation for DC Offset Control
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Solution Overview
Problem
Commercial MEMS microphones with back volumes suffer from significant acoustic noise, limiting their signal-to-noise ratio (SNR), and face challenges with membrane displacement and DC offset due to ambient pressure changes when attempting to create a vacuum environment.
Innovation Solution
A MEMS microphone design featuring a substrate with a back volume, a first and second membrane, and cantilevers that maintain the first membrane's position relative to the sensing device, utilizing pressure relief holes and a flange to prevent DC displacement and enhance SNR, while maintaining a vacuum environment.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a vacuum environment is created in the back volume to eliminate acoustic noise, then the signal-to-noise ratio is improved, but the membrane displacement and DC offset change due to ambient pressure changes
Solution Approach 1:
The back volume is segmented into two separate cavities: a first back volume connected to the sensing device and a second back volume connected to the external environment through pressure relief holes. This segmentation allows the first cavity to maintain vacuum for high SNR while the second cavity compensates for ambient pressure changes, preventing membrane displacement and DC offset variations.
2Volume of stationary object
If the back volume is reduced to minimize acoustic noise, then the device size is reduced, but the noise from the back volume increases
Solution Approach 1:
The back volume is divided into two functional cavities: a small first back volume that maintains vacuum to minimize acoustic noise while providing necessary space for sensing, and a second back volume that manages pressure equalization. This segmentation allows the critical sensing cavity to remain small and quiet while the pressure management cavity handles volume changes.
3Strength
If a high stiffness membrane is used to prevent collapse under 1 atm pressure difference, then the membrane structural integrity is improved, but the microphone sensitivity decreases
Solution Approach 1:
The second back volume acts as a counterweight system by providing pressure compensation through pressure relief holes. This counterbalances the 1 atm pressure difference across the membrane, allowing the use of lower stiffness membranes that maintain both structural integrity and high sensitivity, as the net pressure differential is significantly reduced.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The design effectively reduces noise and maintains stable membrane position, achieving improved SNR and preventing DC offset, enabling high-performance acoustic signal processing in a compact package.
Implementation Method 1
a second membrane provided inside the back volume; second cantilevers, where each of the second cantilevers includes a first end mechanically supporting the first cantilever, and a second end connected to the second membrane
Data Source
AI summary
An MEMS microphone includes a substrate including a back volume provided inside the substrate and an opening provided at an upper surface of the substrate to communicate the back volume; a sensing device provided at an inner side wall of the back volume; a first cantilever provided inside the back volume and including end portions coupling with the sensing device; a first membrane provided at the opening; a second membrane provided inside the back volume; and second cantilevers, each of which includes a first end mechanically supporting the first cantilever, and a second end connected to the second membrane. By suspending the first cantilever on the second cantilevers, the end portions of the first cantilever always couple with a preset position of the sensing device. Thus, the DC offset of the displacement of the membrane can be prevented.